A magnetic disk comprises a substrate, a magnetic recording layer formed on the surface of the substrate, a carbon protective film formed on the surface of the magnetic recording layer, and a film (hereinafter, referred to also as “the surface layer”) formed by applying a lubricant to the surface of the carbon protective film for the purpose of protecting the magnetic disk and read head.
In recent years, along with an increase in the storage capacity of the hard disk drive, the space between the read head and the magnetic disk has become narrower in order to increase the recording density, and the read head and the magnetic disk are likely to get in contact. Therefore, the surface layer is required to be thinner and to be improved in lubricity.
Also, along with an increase in the reading speed of information from the magnetic disk and in the writing speed of information to the magnetic disk, rotation of the magnetic disk has become faster. Therefore, the surface layer is required to be improved in the adhesion to the carbon protective film so as not to scatter from the magnetic disk rotating at a high speed.
As lubricants to form the surface layer, for example, the following ones have been proposed.
(1) a fluorinated polyether compound having four poly(oxyperfluoroalkylene) chains bonded to a tetravalent linking group, wherein each of the chains consists solely of (CF2CF2O) units and has a HOCH2CH(OH)CH2OCH2CF2— group at its terminal (Patent Document 1).
(2) a fluorinated polyether compound having at least three poly(oxyperfluoroalkylene) chains bonded to a trivalent or higher valent linking group, wherein each of the chains consists solely of (CF2CF2O) units, and at least two chains have a HOCH2CF2— group, a HOCH2CH(OH)CH2OCH2CF2— group or the like at their terminals (Patent Document 2).